fix(interpreter): splat "${arr[@]}" elements individually in array assignment

[chaliy]

Summary

• Fix "${arr[@]}" in array assignment producing one joined element instead of individual elements
• Added detection for quoted "${arr[@]}" and "$@" patterns in array context to route through expand_word_to_fields
• combined=("${arr1[@]}" "${arr2[@]}") now correctly produces N+M elements

Test plan

• array_concat_at_expansion — two arrays concat to correct element count
• array_splat_single_source — single "${orig[@]}" copy preserves elements
• All existing array and bash spec tests pass

Closes #963